Field effect transistors (FET) have become the dominant active device for very large scale integration (VLSI) and ultra large scale integration (ULSI) applications, such as logic devices, memory devices and microprocessors, because the integrated circuit FET is by nature a high impedance, high density, low power device. Much research and development activity has focused on improving the speed and integration density of FETs, and on lowering the power consumption thereof.
A high speed, high performance field effect transistor is described in U.S. Pat. Nos. 4,984,043 and 4,990,974, both by coinventor Albert W. Vinal, both entitled Fermi Threshold Field Effect Transistor and both assigned to the assignee of the present invention. These patents describe a metal oxide semiconductor field effect transistor (MOSFET) which operates in the enhancement mode without requiring inversion, by setting the device's threshold voltage to twice the Fermi potential of the semiconductor material. As is well known to those having skill in the art, Fermi potential is defined as that potential for which an energy state in a semiconductor material has a probability of one-half of being occupied by an electron. As described in the above mentioned Vinal patents, when the threshold voltage is set to twice the Fermi potential, the dependence of the threshold voltage on oxide thickness, channel length, drain voltage and substrate doping is substantially eliminated. Moreover, when the threshold voltage is set to twice the Fermi potential, the vertical electric field at the substrate face between the oxide and channel is minimized, and is in fact substantially zero. Carrier mobility in the channel is thereby maximized, leading to a high speed device with greatly reduced hot electron effects. Device performance is substantially independent of device dimensions.
Notwithstanding the vast improvement of the Fermi threshold FET compared to known FET devices, there was a need to lower the capacitance of the Fermi-FET device. Accordingly, in U.S. patent application Ser. No. 07/826,939, now U.S. Pat. No. 5,194,923, and application Ser. No. 07/977,689, both by coinventor Albert W. Vinal, and both entitled Fermi Threshold Field Effect Transistor With Reduced Gate and Diffusion Capacitance, a Fermi-FET device is described which allows conduction carriers to flow within the channel at a predetermined depth in the substrate below the gate, without requiring an inversion layer to be created at the surface of the semiconductor in order to support carrier conduction. Accordingly, the average depth of the channel charge requires inclusion of the permittivity of the substrate as part of the gate capacitance. Gate capacitance is thereby substantially reduced.
As described in the aforesaid patent and patent application, the low capacitance Fermi-FET is preferably implemented using a Fermi-tub region having a predetermined depth and a conductivity type opposite the substrate conductivity type and the same conductivity type as the drain and source. The Fermi-tub extends downward from the substrate surface by a predetermined depth, and the drain and source diffusions are formed in the Fermi-tub within the tub boundaries. The Fermi-tub forms a unijunction transistor, in which the source, drain, channel and Fermi-tub are all doped the same conductivity type, but at different doping concentrations. A low capacitance Fermi-FET is thereby provided. The low capacitance Fermi-FET including the Fermi-tub will be referred to herein as a "low capacitance Fermi-FET" or a "Tub-FET".
Notwithstanding the vast improvement of the Fermi-FET and the low capacitance Fermi-FET compared to known FET devices, there is a continuing need to increase the saturation current per unit channel width which is produced by the Fermi-FET device. Higher current Fermi-FET devices will allow even greater integration density, and/or much higher speeds for logic devices, memory devices, microprocessors and other integrated circuit devices. However, saturation current must not be obtained at the expense of high leakage current. High leakage current dramatically increases the idle power consumption of the device. Finally, as portable and laptop computers proliferate, it is desirable to lower the threshold voltage of a Fermi-FET transistor, without adversely impacting operation thereof. Lower threshold voltage devices are better able to operate with 3 volt or lower power supplies, which are increasingly being used for portable and other energy conscious applications.